Shaped spark drill



March 17, 1970 N. D. SMITH, JR

' sauna SPARK DRILL Filed July 30, 1968 PRESSURE CONTROLS FIG.

CHARGING CIRCUITS FIG.

FIG.

INVENTOR:

N.D. SMITH JR. BY: awn 8:44,

HIS ATTORNEY United States Patent 3,500,942 SHAPED SPARK DRILL Noyes D. Smith, Jr., Bellaire, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed July 30, 1968, Ser. No. 748,802 Int. 'Cl. E21b 7/00 US. Cl. 175-46 6 Claims ABSTRACT OF THE DISCLOSURE A shaped spark discharge for increasing the penetrating power of spark discharges. The spark is forced to jump between a pair of electrodes disposed so that the path along which the voltage is discharged is cone-shaped and causes the shock waves to converge. The electrodes and the surrounding dielectric are shaped and disposed to form a cone-shaped path between the electrodes having the least resistance with respect to an electrical discharge.

BACKGROUND OF THE INVENTION The present invention relates to electrodes for use with spark discharge equipment and more particularly to electrodes that are used in spark discharge equipment designed to create substantial pressure waves, as for example, spark discharges used in drilling earth formations. A spark discharge apparatus of this type is disclosed in Patent 3,158,207 wherein the electrode is positioned in a roller cone drill bit. The spark discharge assists in drilling earth formations, particularly hard formations. The electrodes consists of a solid electrode whose lower end or tip terminates adjacent the lower surface of the drill cones. The drill cones and supporting structure are used as ground electrodes and the discharge occurs between the tip of the first electrode and the drill cones.

While the apparatus disclosed in the above patent combines the benefits of the spark discharge with a conventional roller cone drill bit, it fails to utilize the full po tential of the spark discharge. Since the discharge occurs randomly between the tip of the electrode and the body of the drill bit, a considerable portion of the energy of r the discharge is dissipated in random directions and does not assist in drilling the borehole. Thus, the shock wave produced 'by the discharge is dissipated without accomplishing useful work.

SUMMARY OF THE INVENTION The present invention solves the above problems by providing specially shaped electrodes that concentrate or direct the energy of the discharge toward the target and away from the electrodes. It is well known that when an electric discharge takes place between two electrodes immersed in a fluid, a high-temperature, high-pressure plasma is formed between the electrodes. The expansion of the plasma produces a strong pressure or shock wave that can be utilized to do useful work, as for example, spark discharges have been used as sound sources in seismic work and in metal-forming operations. Likewise, spark discharges have been used experimentally for drilling boreholes in hard earth formations. In all of these operations the discharge has been between a central electrode and a ground electrode and the expanding plasma has generated a substantially spherical shock wave.

The present invention creates a stronger shock wave and directs more of the energy towards the target 'by using an electrode configuration that produces initially a cone-shaped plasma. The cone-shaped plasma generates a converging shock Wave along the axis of the cone and thus produces a higher pressure. The higher pressure produce a stronger shock wave that results in a highspeed jet of fluid being accelerated towards the target.

The shaped electrodes can take various forms with the important feature being that the central electrode and/or the surrounding dielectric material is provided with a conical shape. The open side of the cone is directed towards the target. Preferably, the central electrode is disposed at the apex of the cone with the tip of the electrode terminating flush with the surface of the cone. The second electrode can be either a ground electrode which is disposed outside the cone with its lower end extending below the lower edge of the cone or may be a cylindrical electrode that surrounds the dielectric material. In case of a cylindrical electrode the lower end or tip of the cylinder should extend slightly below the lower end of the cone. In addition to the use of the shaped electrode arrangement, the center electrode can also be pro vided with a fluid passage in order that the electrode can be combined with a normal jet bit. In this arrangement the discharge would serve to break the hard earth formations while the jet would drill the softer formations. In both cases the drill cuttings would be conveyed out of the borehole by the jet fluid. Likewise, the shaped electrode assembly could be combined with a conventional roller cone bit in which the roller cones would insure the drilling of the proper gage borehole while the spark discharge was used to break the hard earth materials.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more easily understood from the following detailed description of a preferred embodiment when taken in conjunction with the attached drawings in which:

FIGURE 1 is a cross section of a shaped electrode assembly constructed according to this invention;

FIGURE 2 is a cross section of a shaped electrode assembly constructed according to this invention combined with a conventional jet bit; and

FIGURE 3 is an elevation view of a borehole showing the shaped electrode assembly of this invention con1- bined with a conventional roller cone bit.

DESCRIPTION OF PREFERRED EMBODIMENTS As explained above when a spark discharge occurs between a pair of electrodes that are immersed in -a liquid, a high-temperature, high-pressure plasma is formed between the electrodes. When the electrodes are conventional rod-shaped electrodes, the plasma will expand in a general spherical form and create a spherical pressure wave as the ionized fluid expands. In a relatively incompressible fluid, as for example, water, the expanding plasma will create a high-energy shock wave that can be used to do useful Work. For example, the shock Wave has been used as a source of seismic energy and in metal forming operations. In addition, the shock wave has been used to drill boreholes either by itself or in combination with other types of drills.

While the above systems of spark discharges have been used as explained above, they are relatively ineffective due to the dissipation of the major portion of the energy in directions other than towards the target.

The present invention is designed to concentrate the energy in the direction of the target. This result is accomplished by providing shaped electrodes that tend to produce initially a cone-shaped plasma. Referring to FIG- URE 1, there is shown one form of the invention which may be incorporated with a conventional jet bit. More particularly, the electrode assembly is secured to the lower end of a drill string 10. The electrode assembly consists of tubular center electrode 11 and an outer or ground electrode 12. The tubular electrode terminates a cone-shaped lower end 13 and is provided with a central bore 14 that serves to conduct the jet fluid to the bottom of the bit. The tubular center electrode serves as the discharge jet and creates a high-velocity jet to drill soft materials. The central electrode is mounted or embedded in an insulator formed of dielectric material which may be conventional molded plastic or other suitable dielectric material. The upper end of the outer electrode is provided with male threads in order that the electrode assembly may be secured to the threaded end of the drill string. The bottom end 17 of the insulator terminates adjacent the outer periphery of the conical surface 13 of the central electrode. The lower end or tip of the ground electrode 12 ends above the lower end of the insulator shown in FIGURE 1. Conductors 18 and 19 are provided for coupling the two electrodes to the remainder of the circuitry, not shown. Of course, the outer electrode 12 can be coupled to the drill string 10; the drill string could act as a ground for the circuit.

The above shaped spark source is operated by applying a suitable voltage to the central electrode. This can be accomplished by discharging a storage capacitor across the electrodes. The discharge of the voltage will cause a spark to be initiated between the center electrode and the conducting drilling fluid connected to ground electrode 12. The spark, of course, ionizes a portion of the fluid between the surrounding electrodes and produces a hightemperature, high-pressure plasma. The resulting plasma generates a converging shock wave (shown by lines 9) that tends to converge along the axis of the cone, producing a high-speed jet of fluid. The resulting jet of fluid is accelerated by the converging shock wave and directed towards the bottom of the borehole. Thus, the maximum amount of energy of the plasma is directed towards the bottom of the borehole.

Referring now to FIGURE 2 there is shown a modified form of the invention which is particularly adapted for use with a jet bit. More particularly, the modified form uses a tubular central electrode 20 Whose lower end terminates at the apex of the conical surface 22 of the insulator 21. A tubular outer electrode 23 is provided with male threads for attaching the assembly to the bit or drill string, e.g., via the bit-body. The lower ends of the outer electrode 23 terminates adjacent the periphery of the conical surface 22. A conductor 24 is disposed in the insulator and couples the electrode 20 to circuitry not shown.

The modified electrode assembly can be operated as a jet bit by jetting a suitable fluid through the center electrode when the drill is operating in soft material. It can also be operated as a spark drill in hard material by discharging a suitable voltage between the electrodes or as a combination spark and jet drill.

For use in fresh water of low conductivity the lower portion 39 of insulator 21 may be made of a cation exchange resin such as Dowex 50W, sold by Dow Chemical Company of Midland, Mich., dispersed in a plastic such as nylon. Such a material has a very low bulk conductivity, but a surface exposed to water acquires a relatively high conductivity which provides a conducting path to initiate the discharge.

Referring now to FIGURE 3, there is shown the electrode assembly combined with a conventional roller cone bit. More particularly, there is shown a borehole having a drill string 31 extending downwardly therein. The bottom end of the drill string is coupled to a tool section 32 with the roller cone bit 33 being secured to the lower end of the tool section. A shaped electrode assembly similar to that described in FIGURE 2 is secured to the center of the drill bit assembly.

The tool section 32 contains the equipment for operating the shaped spark electrode assembly and may include a mud turbine 36 and suitable pressure controls 35. The

4 37 that provides a suitable voltage for the charging circuits 38. The charging circuits in turn are coupled to the electrode assembly and include suitable controls for controlling the operation of the shaped spark source.

The combination of the roller cone bit and shaped spark source provides a drill that is capable of boring a true gauge hole in formations. More particularly, the roller cones will insure the true gauge of the hole while the shaped spark discharge will supply the high power required to break hard formations. Thus, the combination provides an efficient means for drilling earth formations. Of course, the roller cones may be used to drill earth formations without resorting to the use of a spark discharge.

From the above description it is appreciated that this invention provides a means for increasing the effectiveness of a spark type drill Without increasing the power supplied to the drill. This is achieved by providing a coneshaped end on the electrode assembly to generate a large shock wave and direct it toward the desired target. By concentrating the shock wave instead of allowing it to disperse in the form of a radial shock wave, the efficiency of the discharge is improved. More than one spark can be disposed at the end of a drill string or on one bit. Such a spark could be operated simultaneously or sequentially in varied programs.

I claim as my invention:

1. An electrode assembly for a spark discharge for concentrating the spark discharge along an axis, said assembly comprising:

a first electrode, said first electrode being disposed along said axis;

a second electrode, said second electrode being displaced from said first electrode;

an insulator, said insulator surrounding said first electrode; and

one of said first electrode and said insulator having a conical shape with the axis of the cone being coaxial with said axis.

2. The assembly of claim 1 wherein said first electrode terminates in a conical-shaped end.

3. The assembly of claim 1 wherein said first electrode is provided with a fluid passage through the center thereof.

4. The assembly of claim 3 wherein said second electrode surrounds said first electrode with said insulator being disposed between said first and second electrodes, the portion of said insulator adjacent said electrodes having conical shape, the end of said second electrode being adjacent the outer periphery of the insulator.

'5. An apparatus for drilling a borehole comprising:

a drill string;

a tool section attached to the lower end of said drill string;

a drill bit secured to the lower end of the tool section;

and

a shaped spark discharge disposed on said drill bit to direct the spark discharge downwardly, said spark discharge comprising a central electrode surrounded by an insulator and an outer electrode, the lower end of one of said electrode and insulator having a recessed conical shape.

6. The apparatus of claim 5 wherein said insulator is formed of a material that exhibits a relatively greater conductivity in contact with water.

References Cited UNITED STATES PATENTS 1,898,926 2/1933 Harts et al. 16 2,772,346 11/1956 Leston et al. 175-16 2,822,148 2/1958 Murray 175-16 X 3,122,212 2/1964 Karlovitz l7516 X 3,179,187 4/1965 Sarapuu 175-16 DAVID H. BROWN, Primary Examiner 

